1. Field of the Invention
The invention generally relates to a vibratory compactor used, e.g., to compact backfilled trenches after a pipeline is laid or to compact the floor of a trench or to compact asphalt or larger areas, and more particularly, relates to a vibratory compactor of the above-mentioned type having an electric drive.
2. Discussion of the Related Art
Vibratory compactors are used in a variety of ground compaction and ground leveling applications. Most vibratory compactors have plates or rollers that rest on the surface to be compacted and that are excited to vibrate so as to compact and level the worked surface. A common vibratory compactor, and one to which the invention is well-suited, is a vibratory trench roller.
The typical vibratory trench roller includes a chassis supported on the surface to be compacted by one or more rotating drum assemblies. Two drum assemblies are typically provided, each of which may support a respective subframe of the chassis if the trench roller is an articulated trench roller. The subframes may be articulated to one another by a pivot connection. Each of the drum assemblies include a stationary axle housing and a drum that is mounted on the axle housing and that is driven to rotate by a dedicated hydraulic motor. Both hydraulic motors are supplied with pressurized hydraulic fluid from a pump powered by an internal combustion engine mounted on one of the subframes. In addition, each drum is excited to vibrate by a dedicated exciter assembly that is located within the associated sub-frame and is powered by a hydraulic motor connected to a pump. The exciter assembly typically comprises one or more eccentric masses mounted on a rotatable shaft positioned within the sub-frame. Rotation of the eccentric shaft imparts vibrations to the sub-frame and to the remainder of the drum assembly. The entire machine is configured to be as narrow as possible so as to permit the machine to fit within a trench whose floor is too compacted. Machine widths of less than 3 feet (1 meter) are common. Vibratory trench rollers of this basic type are disclosed, e.g., in U.S. Pat. Nos. 4,732,507 to Artzberger, 5,082,396 to Polacek, and 7,059,802 to Geier et al., the entireties of which are hereby expressly incorporated by reference thereto.
The hydraulic systems of vibratory trench rollers of the kind generally known in the art are configured to control the functions thereof including forward and reverse travel, steering, and vibratory excitation. Hydraulic power is produced by hydraulic pumps connected to the engine. Pressurized fluid from the pumps is routed by a hydraulic manifold to the hydraulic motors and cylinders to control the operations of the machine. Low-speed hydraulic motors drive the drums through a gear reduction, and vibratory excitation is generated by a hydraulic motor driving eccentric shafts at high speeds. Hydraulic fluid, typically oil, flows through a heat exchanger and a filter prior to returning to the reservoir in order to maintain system performance and reduce wear on the hydraulic components.
The typical hydraulic systems, though adequately operating and carrying out the functions of the machine, exhibit several drawbacks and disadvantages. First, as in any hydraulic system, there is the potential for leaks at any connection point along the system. The amount of vibratory excitation present in trench rollers of the kind under consideration herein only exacerbates this problem. Over time, the vibrations experienced can cause the hydraulic fittings to loosen and the hoses to fail from abrasion with other components and/or hoses. In the case of a hydraulic fluid leak, the roller may cease to operate and/or hydraulic fluid may leak onto and contaminate the surrounding soil.
In addition, hydraulics are inefficient as compared to other types of power transfer. Such system inefficiencies result in an undesirable amount of heat generation which is transferred through the fluid to the other hydraulic components in the system. This heat must be eliminated so as to prevent damage to the components of the machine, which adds to the complexity, cost, and inefficiencies of the overall system.
Moreover, the hydraulic valves necessary to control the flow of the hydraulic fluid through the system are quite costly. Many different valves are required to perform the functions required of vibratory trench rollers thus substantially increasing the costs associated with the production of such machines. Further, simple hydraulic controls act in an on-off manner. Thus, the flow of hydraulic fluid to components is generally started and stopped very quickly. Relief valves are inserted into the system to limit the pressures generated by these quick changes to flow. As noted previously, valves are quite costly. The additional relief valves add to the cost of the machine. Further, the addition of a number of components such as relief valves only increases the number of elements capable of failure and requiring maintenance or replacement. Hydraulic functions could be activated in a more controlled manner using proportional valves. However, such valves are even more expensive and require more complicated control systems to drive them so they are generally not cost effective for vibratory trench rollers and similar machines.
As noted above, simple hydraulics operate in an on-off manner and create high pressure spikes during transition. The high pressure conditions last only a short time (under 2 seconds) but the engines that power hydraulic systems must be sized so that the engine does not bog down under maximum power draw, such as occurs when engaging the exciter while traveling up a slope. If a machine seldom operates under these conditions, as is often the case for vibratory trench rollers, the engine operates at less than peak efficiency the vast majority of the time. In other words, the engine needs to be considerably oversized so as to be capable of meeting relatively infrequent but steep spikes in demanded power. A larger engine, of course, also costs more and requires more fuel.
Finally, hydraulic hoses must be sized according to the flow requirements of the system. These hoses can measure more than one-inch in diameter. The coverings for the hoses are generally constructed to resist abrasive wear, which makes it difficult to bend or otherwise manipulate the hoses. As such, it is rather difficult to route multiple hoses in a relatively confined space.
The foregoing drawbacks and disadvantages result in a number of system shortcomings and failures including, but not necessarily limited to, hydraulic leaks caused by loose-fitting or damaged o-rings, exciter motor shaft seal failures or housing cracks, hose abrasion damage, hydraulic manifold leaking, and/or cartridge valve failure. Further, such system failures commonly occur in and affect the components inside the sub-frames and these issues are often time-consuming and therefore costly to remedy. The compact design of the rollers requires that the components thereof be placed in tight locations that are often blocked or impeded by other components of the roller. As such, it can be quite difficult to determine the location of and repair a leak.
Other types of vibratory compacting machines employ similar hydraulic drives and suffer from the same or similar drawbacks heretofore described. In addition, certain other types of vibratory compacting machines, such as ride-on rollers used for compacting soil or smoothing asphalt, also suffer from additional drawbacks.
For example, the hydraulic systems of ride-on rollers have a number of inefficiencies that require these rollers to use an engine large enough to operate all of the systems of the roller at peak pressures. For instance, the exciter systems of ride-on rollers are usually controlled with simple on-off hydraulic valves that start and stop the flow of hydraulic fluid to the exciter motor very quickly. Rapidly accelerating the exciter mass from stop to the rated operation speed requires a large amount of torque. Once the exciter is at operating speed, the torque requirements are greatly reduced. Torque is generated when the high pressure hydraulic fluid from the pump attached to the engine flows through the hydraulic motor. High pressures and high flows require more power from the engine.
In addition, ride-on rollers usually do not require the full torque of the drive system during use. High torque is required only when operating on steep hills, loading or unloading from a trailer, or when the machine operates in loose soil. This high torque may be required from 1-50% of the duty cycle depending on the specific application. Thus, the engine must be sized to meet these peak pressure and flow demands. However, as with trench rollers and other compactors, such high-load operating conditions are present for only a limited amount of the operational time, which may be as low as 1%. The extra engine power capacity therefore is seldom used. By requiring a larger engine for what amounts to a small fraction of the time of the overall operation of the machine, the overall size, weight, and cost of the roller is greatly increased.
Further, the drive systems used in modern ride-on rollers are typically also hydraulic, but these drive systems are different than those used in trench rollers. Ride-on rollers use a hydrostatic pump that is able to proportionally control the flow rate of the hydraulic fluid of the pump. These pumps provide variable speed and eliminate the on-off nature of the simple hydraulic valves. However, hydrostatic pumps are less efficient and also operate as a so-called “closed loop” system that can require additional measures for removing heat to avoid component damage.
Many hydrostatic drive systems for ride-on rollers are comprised of two parallel loops, one for the front drum and one for the rear drum. The hydraulic fluid in these systems flows to the path of least resistance so if one drum loses traction it will get all of the flow. A flow divider is sometimes used on these machines to provide so-called “traction control” for these situations. Flow dividers create additional heat and add to the complexity and cost of the roller. Hydrostatic pumps are also directly coupled to the engine, so they are constantly being driven, creating a parasitic load on the engine even when the machine is not moving. Finally, such hydrostatic drive systems are relatively expensive.
In addition, ride-on rollers typically are used in a cyclical manner, i.e. driving back and forth over a section of soil or asphalt to compact the surface. The cyclical operation of the machine requires energy to accelerate and decelerate the machine as it changes direction. The cyclical operation of the machine can also create varying levels of power required to drive the system, i.e. compacting material on a slope will require more power to drive up the slope than to drive down.
The need therefore exists to provide a drive system for a vibratory roller of the like that eliminates one or more of the foregoing disadvantages.